1. Field of the Invention
The present invention relates generally to a device for alleviating the downhole forces exerted on a drill bit in order to prevent the bit from stalling and methods of drilling a well bore in a subterranean formation using a coiled tubing, or other conventional or slim hole (having low torque limits), drill string comprising the device. Particularly, the device utilizes the pressure differential between the drilling fluid flowing downhole towards the drill bit and the fluid in the annulus proximate the device to reduce the downhole force on the drill bit.
2. Description of the Prior Art
Coiled tubing (CT) drill strings present certain advantages over traditional, rigid-pipe drill strings, particularly in their ability for conducting directional drilling in under balanced or pressure managed drilling operations. For example, a CT string can initially drill a vertical well bore to a desired depth and then change directions and continue to drill at an oblique angle to the previously drilled well bore section. The ability to control the angle or direction of the drill bit is essential to directional drilling operations.
Stalling of the drill bit is a problem that can be encountered with CT or other slim hole drilling operations. Stalling occurs when the downhole force on the drill bit becomes so great that the mud motor can no longer turn the bit. CT drill strings are specially susceptible to stalling because as the internal pressure within the string increases, the tubing or slack in the tubing may slip downhole causing the CT and bottom hole assembly to surge forward. This forward surge places an additional demand on the mud motor that further increases the internal pressure within the string.
Previously, this problem was addressed by locking the drill string in place several inches or feet above the face of the formation and then waiting for the CT slack to work down thereby minimizing the surge elongation. However, this practice can be very tedious and time consuming often adding hours to the drilling operation. Anti-surge tools have been proposed to combat this problem. However, such prior devices operate by pulling the drill bit out of contact with the formation and is often accompanied by some rotation of the bottom hole assembly, or portion thereof, relative to the CT. This rotation is undesirable as it can affect the direction of the drill bit and lead to drilling in an unintended direction. Thus, there exists a need for a downhole tool that overcomes the above problems associated with stalling of the drill bit during coiled tubing or other conventional slim hole drilling operations.